In recent years, in the field of automobile industry and advanced electronic industry, the demand for secondary batteries such as automobile batteries and electronic devices is increasing. In these secondary batteries, an exterior or outer member made of a flexibly deformable laminate sheet seals a unit cell which is a battery element. Furthermore, a battery module is structured by enclosing a cell unit including a laminate of several such unit cells stacked on one another by upper and lower cases. Moreover, by connecting multiple battery modules configured in this way in series or in parallel, a battery pack is formed, which corresponds to a desired current, voltage, or capacity (for example, see JP 2003-203615 JP).
In the secondary battery described in JP 2003-203615 JP, however, since the height of a pair of side surfaces of the upper and lower covers are the same, when accommodating the cell unit in the lower cover at the time of assembly, or when covering the cell unit with a top cover, it is necessary to adjust the battery module, the upper cover, or the lower cover to find a position for proper fitting. Therefore, it takes a long time for assembly. Also, there is a concern that the cell unit is damaged due to interference to of the cell unit with the side surface.